The semiconductor detector used for a radiation detector has a higher energy resolution on an X-ray and a gamma ray than the scintillation detector, which is an excellent feature for the semiconductor detector. As a result, semiconductor radiation detectors of Silicon or Germanium are often used to accurately measure energies of X-rays and gamma rays. However, since Silicon has an atomic number of 14 which is relatively small and a density about as small as 2.3 g/cm3, Silicon is not capable of absorbing high energy X-rays and gamma rays efficiently. Moreover, although Germanium has an atomic number of 32 that is larger than that of Silicon and a density of 5.4 g/cm3 that is larger than that of Silicon, Germanium has a band gap as small as 0.7 eV and should be used while kept cooled at a temperature as low as that of liquid nitrogen.
On the other hand, semiconductor detectors, which are capable of absorbing high energy X-rays and gamma rays efficiently and used at room temperatures, have been developed and are cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) which are typical among them. For instance, CdTe having an atomic number of 50 and a density of 5.8 g/cm3 is capable of absorbing high energy X-rays and gamma rays more efficiently than Germanium and is used at room temperatures because of having a larger band gap of 1.4 eV. However, such compound semiconductor materials as CdTe and CdZnTe have the following problem. That is, mobilities of an electron and a hole in a carrier crystal of any of CdTe and CdZnTe is smaller than those in Si or Ge. When an semiconductor element is used for a radiation detector, carriers are generated according to a photoelectric effect on gamma ray coming incident into the semiconductor element. These carriers are measured by measuring an electrical current of the carriers flowing through an external circuit. Therefore a product of the mobility μ and its duration of life τ is used for a criterion to determine whether a semiconductor detector, is sufficiently good.
In general, the larger the product of μ and τ, the higher the energy resolution becomes. If the product of μ and τ is small, the energy resolution becomes low. Since CdTe and CdZnTe have smaller products of μ and τ than that of Si or Ge, the energy resolution is lower for CdTe and CdZnTe than for Si and Ge. Therefore CdTe and CdZnTe are not suited for measuring accurately energies of X-rays or gamma rays. However, for example, if the semiconductor detector of CdTe and CdZnTe is utilized instead of the scintillation detector for a gamma camera, a nuclear medicine diagnosis apparatus such as single photon emission computed tomography (SPECT) and a radioactive substance detection apparatus used outdoors, for each of which the scintillation detector has been used so far, each of the apparatuses could have a higher energy resolution and an improved function.
Such materials as CdTe or CdZnTe have smaller products of μ and τ than that of Si or Ge, as already explained. Especially the products of μ and τ of the holes of CdTe and CdZnTe are smaller, which is a problem. The product of μ and τ of the hole of CdZnTe is smaller than CdTe. To overcome the problem of the smaller product of μ and τ of the hole, an improved electrode construction and an improved circuit with a modified read-out circuit, which is disclosed by the non-patent document 1, are used. Although this method is excellent because signals are read only through electrons, it is necessary to divide an anode into small electrodes and make a distance between the small electrodes of the anode and a cathode and an anode larger in order to have hole induced charges having a less effect on the electrodes. Therefore this method is suited for a relatively large crystal and properties of the crystal need to be uniform in the entire crystal. On the other hand, it is difficult to produce a large crystal in which properties are uniform.
Since the product of μ and τ of the hole of CdTe is not so small as that of CdZnTe, CdTe is used usually with planar plate type electrodes (Planar type) which is much simpler than the complicated electrodes for CdZnTe. However, when the distance between the electrodes, which corresponds to a thickness of the element, becomes large, the energy resolution becomes lower due to the effect from the small product of μ and τ of the hole. Accordingly relatively thin elements of CdTe having thickness of 0.5 to 1.0 mm are usually used.
The method disclosed in the patent document 1 makes use of a couple of waveform shaping circuits whose time constants are different from each other. This method performs a correction assuming the product of μ and τ of the hole being small, and the energy resolution is expected to be high. A specific correction method, which is disclosed in the non-patent document 2 and the non-patent document 3, makes use of Vf/Vs where Vf is an output from the fast speed waveform shaping circuit and Vs is an output from the slow speed waveform shaping circuit. The non-patent document 4 discloses a basic principle of a correction method called a biparametric correction, which makes use of a difference between an increase in the induced charges with the electron and an increase in the induced charges with the hole.